1. Field of the Invention
The present invention relates generally to ink-jet printing and, more specifically, to print media edge detection using an optical sensing device.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
In essence, the ink-jet printing process involves dot-matrix manipulation of drops of ink ejected from a pen onto an adjacent print medium (for convenience of explanation, the word xe2x80x9cpaperxe2x80x9d is used hereinafter as generic for all forms of print media). An ink-jet pen includes a printhead which consists of a number of columns of ink nozzles, with each typically having a diameter of only about {fraction (1/300)}th inch. A column of nozzles (typically less than or equal to one-inch in total height) selectively fires ink droplets (typically only a few picoliters in liquid volume) that are used to create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned across the media. A given nozzle of the printhead is used to address a given matrix column print position on the paper, referred to as a picture element, or xe2x80x9cpixel.xe2x80x9d Horizontal positions on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of dots. The paper is stepped to permit a series of contiguous swaths. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops.
Paper position during printing is of course a prime factor to the quality of the print. Two basic types of prior solutions to determining the side edge of the media are interrupters and edge guides. An xe2x80x9cinterrupter,xe2x80x9d whether optical or capacitive type, is mounted in a known, fixed position relative to the nominal paper movement. The interrupter switch binary output merely tells if the media edge is present within the field of view of the interrupter. Edge determination is made by sampling a paper advance axis encoder and the interrupter state. However, this gives no direct data with regard to the edge""s real time position relative to the printing zone of the printer and its writing instrument or plurality of writing instruments. The second type, known as xe2x80x9cedge guidesxe2x80x9d are positioned relative to the paper edge by the printer user. Print media edge detection relies on the known printer""s paper width and length by using adjusters to mechanically position the paper sheet relative to the hard copy apparatus printing zone and writing instruments. Edge position is inferred from the adjustment. For example, U.S. Pat. No. 5,574,551 (Kazakoff) for a PRINT MEDIA SUPPLY APPARATUS WITH MEDIA PARAMETER DETECTION CAPABILITY (assigned to the common assignee of the present invention and incorporated herein by reference) provides for size detection in the paper supply bin.
The position of the paper transported to a printing station of the hard copy apparatus must be known or determined for the droplets of ink to be appropriately registered on the paper. The more detailed the print, e.g., a photographic or art reproduction, the more critical the drop placement in order to avoid noticeable artifacts.
U.S. Pat. No. 5,466,079 (Quintana) shows a APPARATUS FOR DETECTING MEDIA LEADING EDGE AND METHOD FOR SUBSTANTIALLY ELIMINATING PICK SKEW IN A MEDIA HANDLING SUBSYSTEM and its divisional U.S. Pat. No. 5,564,848 (Quintana) shows a METHOD AND APPARATUS FOR DETECTING MEDIA SHEET EDGES WITH A COMMON, MOVABLE OPTICAL SENSOR (each assigned to the common assignee of the present invention and incorporated herein by reference). An electro-optic sensor detects when the top of a media sheet enters between a drive roller and a pinch roller and the media is manipulated to be squared up.
In U.S. Pat. No. 5,252,991 (Storlie et al.) (assigned to the common assignee of the present invention and incorporated herein by reference) for a MEDIA EDGE SENSOR UTILIZING A LASER BEAM SCANNER, a scanning system moves the light beam across the media sheet and past its edges to cause the beam to fall on beam sensor when the media sheet is partially shadowing the beam sensors. A processor is responsive to outputs from the beam sensors to determine positional information regarding the media sheet. Positional information is derived by determining the time during which the scanned beam is incident on a sensor. In U.S. Pat. No. 5,446,559 (Birk) (assigned to the common assignee of the present invention and incorporated herein by reference), a handheld scanner is moved over the edge of the printed page to calibrate the position of the scanner with respect to the sheet of paper.
The state of the art devices leave some degree of uncertainty as to the real time position and skew of the paper once transported to the printing zone and while being stepped through the printing zone. This is particularly problematical in full-bleed ink-jet printing, i.e., printing to or very near the edges, such as might be desirable for photographic reproductions. Thus, there remains a need for making real time determinations of paper edge position when the paper sheet is in the hard copy apparatus printing station.
In its basic aspects, the present invention provides a method for detecting print medium edge position in a hard copy apparatus having a printing zone including a platen having a width greater than a predetermined maximum width for print media used with the hard copy apparatus. The method includes the steps of: transporting a sheet of the print media into the printing zone for printing on the sheet with a writing instrument; scanning across the printing zone with an optical sensing device having a known field of view; while scanning, recording a series of reflectance readings from the sheet and from the platen at a platen region adjacent at least one edge of the sheet; storing the reflectance readings as a first data set; calculating a second data set based on the first data set and the known field of view; calculating a best fit of the second data set to the first data set; and determining a reflectance transition point location in the printing zone of the best fit wherein the transition reflectance transition point is representative of the print media sheet edge position.
It is another basic aspect of the present invention to provide an ink-jet hard copy apparatus adapted for using a variety of print media, including: a printing station within the apparatus, the printing station fabricated of a material having a predetermined first reflectivity recognizably differing from reflectivity of blank print media, the printing station having a width greater than a predetermined width equal to a maximum usable print media width; a scanning carriage mounted within the apparatus for selectively scanning across the printing station width; an encoder system associated with the carriage for tracking position of the carriage; a plurality of printheads fixedly mounted to the carriage; an optical detector fixedly mounted to the carriage in a predetermined relationship to the printheads, the detector having a known field of view; and a control mechanism for selectively determining true edge position of at least one edge of a sheet of print medium positioned at the printing station by comparing a reflectivity data set obtained with the optical detector from the sheet and from the material by scanning the detector across the sheet a distance greater than a nominal width of the sheet to obtain positionally related reflectivity values from both the blank print media and the material to a calculated data set constructed from the reflectivity data and the predetermined field of view and for calculating the true sheet edge position therefrom.
In another basic aspect, the present invention provides a hard copy apparatus, including: holding mechanisms for holding a sheet of print media in selective orientations while printing is performed thereon; at least one writing mechanism for printing associated with the mechanisms for holding; mounting mechanisms for fixedly mounting the writing mechanisms thereon and for selectively positioning the writing mechanisms with respect to the holding mechanisms; tracking mechanisms for tracking predetermined positions of the mounting mechanisms while the mounting mechanism is selectively positioning the writing mechanisms; fixedly mounted on the mounting mechanisms, detecting mechanisms for determining reflectivity at the predetermined positions within the holding mechanisms, including across an edge of the sheet in a first direction of travel of the mounting mechanisms, the detecting mechanisms having known operating parameters; and controlling mechanisms for compiling a first data set from the detecting mechanisms, the first data set being representative of actual reflectivity values, including reflectivity values of the sheet and of the holding mechanisms; and calculating mechanisms for calculating a second data set representative of data compiled by the detecting mechanisms and of the known operating parameters of the detecting mechanisms and for comparing the second data set to the first data set such that a best fit of the second data set to the first data set provides a transition value representative of position of the edge of the sheet.
In another basic aspect, the present invention provides a method for ink-jet printing with, including the steps of: a) providing at least one ink-jet writing instrument fixedly mounted in a hard copy apparatus for scanning across a print media platen; b) providing at least one optical scanning device, having known operating characteristics, fixedly mounted in a known relationship to the writing instrument for scanning across the print media platen therewith; c) transporting a sheet of print media to a printing zone on the platen wherein the sheet has a predetermined nominal position on the platen; d) optically scanning across a region of the sheet near a first edge thereof with the optical scanning device while recording media reflectivity data therefrom with respect to known positions across the platen; e) optically scanning across the first edge of the sheet while recording reflectivity transition data between the sheet and the platen; f) optically scanning across a region of the platen while recording platen surface reflectivity data therefrom; g) calculating a true position of the edge of the sheet from the recording media reflectivity data, the reflectivity transition data, the platen surface reflectivity data, the known operating characteristics of the optical scanning device, and the optical scanning device known relationship to the writing instrument; and h) printing images with the writing instrument relative to the true position of the edge.
In another basic aspect, the present invention provides a computer memory device including: mechanisms for recording a series of reflectance readings from the sheet and from the platen adjacent at least one edge of the sheet; mechanisms for storing the reflectance readings as a first data set; mechanisms for calculating a second data set based on the first data set and the known field of view; mechanisms for calculating a best fit of the second data set to the first data set; and mechanisms for determining a reflectance transition point location in the printing zone of the best fit wherein the transition reflectance transition point is representative of the print medium edge position.
It is an advantage of the present invention that it provides a method and apparatus for optically determining print medium edge position in real time with a print medium in the printing zone.
It is an advantage of the present invention that measurement of print medium edge position at differing points during paper advance produces a measurement of print medium skew.
It is another advantage of the present invention that it is sufficiently fast to calculate position while simultaneously printing adjacently to an edge, enabling full-bleed printing adjustments.
It is a further advantage of the present invention that it does not require any optically detectable markings to be made on the print medium.
It is an advantage of the present invention that it provides a solution that does not require additional edge detector components to an ink-jet hard copy apparatus.
It is another advantage of the present invention that by using existing product devices manufacturing costs are not increased.
It is still another advantage of the present invention that it can be used to measure accumulative tolerance with respect to the print medium, enabling loosening of mechanical part tolerances and thus lowering manufacturing costs.
It is another advantage of the present invention that by using existing product devices it reduces the potential of added complexity lowering reliability.
It is a further advantage of the present invention that it is applicable to all hard copy apparatus platforms.
It is a further advantage of the present invention that it is a relative measurement, requiring no calibration.
It is a further advantage of the present invention that it is implementable automatically as a programmed algorithm, requiring no end user intervention.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.